I hope you see where I am going here. Tests rarely give a yes/no answer. Results have to be interpreted in the light of the patient's story. That's why we have doctors. I'm going to tell you two stories I heard recently about the spectacular misuse of d dimer.
A 50-year-old man was clearing his gutter when he disturbed a wasps' nest. In the surprise and chaos that ensued, he fell off his ladder and got stung several times, bruising his hip in the process. He was brought to hospital by ambulance. To keep waiting times down, patients in UK Emergency Departments are usually seen by a triage nurse immediately on arrival and have bloods taken while waiting to see a doctor. In this case, the patient had a d dimer test and it was significantly raised. He was admitted to the Acute Medical Unit, seen by a junior doctor and listed for a CT pulmonary angiogram (a test for a suspected blood clot on the lung). Now, even lay people will ask themselves at this point: how has falling off a ladder and being stung by wasps been interpreted as a blood clot on the lung? - a condition that presents with unexplained breathlessness and/or spontaneous chest pain. Luckily this man was intercepted by a chest physician who cancelled the test.
The second patient, also a 50-year-old man, attended the Emergency Department with fever and earache. For some reason, his blood tests included a d dimer which was significantly raised. He was scheduled for a CT pulmonary angiogram which was normal - apart from a few slightly enlarged lymph nodes in the chest. His case was discussed with lymph node specialists and for completeness he went on to have a CT scan of his abdomen and pelvis which was normal. The lymph nodes were deemed insignificant - what we call an 'incidental finding'. The cost of the CT scans would have been around £600, and 3 days in hospital around £675. The impact of radiation exposure on patients from CT scans of the body is uncertain, but according to the US Food and Drug Administration, it is not much less than the lowest doses received by some of the Japanese survivors of the atomic bombs (who have a small excess cancer risk). By the way, no-one looked in his ear until day 3 when his ear infection was diagnosed.
I have changed some of the details of these stories for confidentiality reasons, but there are many more stories about the use and abuse of d dimer I could tell. It has been estimated that 20% of healthcare expenditure is waste. Lots of waste in the form of indiscriminate testing could be minimised by simply taking time to listen to the patient's story, don't you think?
Preventing overdiagnosis: how to stop harming the healthy. BMJ 2012; 344: e3502